It is known that chromium-based catalyst systems with diphosphine ligands catalyse the selective conversion of ethylene to 1-hexene and/or 1-octene, depending on the reaction conditions and choice of ligand structure. In particular, the nature and position of any substituents on the aryl rings connected to the phosphines are crucial influences on the selectivity towards tetramerisation of ethylene. By tetramerisation it is meant that at least 30% 1-octene is produced in the process.
Non-limiting examples of selective ethylene tetramerisation catalyst systems include the ubiquitous Cr/bis(phosphino)amine (i.e. ‘PNP’) systems, particularly of the type (Ar1)(Ar2)PN(R)P(Ar3)(Ar4), where Ar1 to Ar4 are aryl groups such as phenyl and R is a hydrocarbyl or a heterohydrocarbyl group, beginning with PNP ligands containing no substituents on the phenyl rings bonded to the P-atoms (e.g. as described in WO 2004/056479) and those with m- or p-methoxy groups on the phenyl rings (e.g. as described in WO 2004/056480). In addition to this, PNP systems containing o-fluoro groups on the phenyl rings are described in US 2008/0242811 and US 2010/008177, and PNP systems bearing pendant donor atoms on the nitrogen linker are described in WO 2007/088329. Multi-site PNP ligands are discussed in US 2008/0027188. In addition to the Cr/PNP systems, chromium systems bearing N,N-bidentate ligands (e.g. as described in US 2006/0247399) can be used. PNP ligands with alkylamine or phosphinoamine groups bonded to one of the PNP phosphines (i.e. ‘PNPNH’ and ‘PNPNP’ ligands) are described in WO 2009/006979. Finally, carbon bridged diphosphine (i.e. ‘PCCP’ ligands) are described in WO 2008/088178 and WO 2009/022770.
A serious drawback for tetramerisation catalysts generally is the low catalyst activity when operated at elevated temperatures, especially above 80° C. This may be explained in some cases by catalyst deactivation at elevated temperatures as described in Applied Catalysis A: General 306 (2006) 184-191.
In a recent review article describing catalyst systems for ethylene tetramerisation, van Leeuwen at al (Coordination Chemistry Reviews, 255, (2011), 1499-1517) have discussed the problems associated with elevated reaction temperatures. They state that: “In general the selective ethylene tetramerisation experiments are performed in the temperature range 40-60° C. Various studies on both semi-batch and continuous mini plant have shown a strong dependency of the reaction temperature on the activity and selectivity of the Cr(III)/Ph2N(R)PPh2/MAO catalytic system. High reaction temperatures (>60° C.) significantly reduced the catalyst productivity as compared to reactions performed at lower temperature under the same ethylene pressure. Consequently catalyst decomposition with increasing temperature is probably the main reason for lower productivities at high temperatures.
When carrying out a process for tetramerisation of ethylene, the aim is to choose a catalyst system and adjust process conditions in order to produce the maximum amount of 1-octene, as opposed to trimerisation processes where catalysts and process conditions are adjusted to produce the maximum amount of 1-hexene. 1-Hexene is also typically co-produced in a tetramerisation process and it is well known in the art of the invention that higher temperatures shift the selectivity from 1-octene towards 1-hexene. This is a further issue to consider when operating a tetramerisation process at higher temperatures.
Furthermore, the formation of a high molecular weight polymer co-product by the Cr-based ethylene tetramerisation catalyst may present a major technical challenge when commercialising an ethylene tetramerisation process as polymer fouling reduces plant run time and necessitates shut-downs due to blockages and difficult temperature control. When running tetramerisation processes at reaction temperatures in the range of 40 to 80° C., the polymer precipitates out of solution in the reactor, which brings risk to the process due to the possibility of reactor or downstream equipment fouling.
Running a tetramerisation process at process conditions where the polymer co-product remains predominantly dissolved in the liquid reaction medium in the reactor (i.e. a solution phase process) would substantially reduce the possibility of reactor or downstream fouling. For example, in the field of polymerisation, solution-phase polyethylene processes are used by Dow Chemicals (Dowlex™ process) and Nova Chemicals (Sclairtech™ and Advanced Sclairtech™ processes) to produce high value linear low density polyethylene products (Linear Low Density Polyethylene, Process Economics Program Report 36E, Susan Bell, August 2008). However these processes run at reaction temperatures of about 150° C. to 300° C., such high temperatures being required to keep the polyethylene product in solution.